The invention relates to a two-stroke engine including a mixture lubricated two-stroke engine for a portable handheld work apparatus, such as a motor-driven chain saw, cutoff machine, blower apparatus, brushcutter or the like.
A two-stroke engine of the above kind is disclosed in U.S. Pat. No. 6,216,650. The bypass channel, which supplies clean air, opens via a membrane valve into the transfer channel which is configured as a radially open channel. The radial opening of the transfer channel is closed by a valve housing which carries the membrane valve and which is to be mounted on the cylinder. This requires a significant manufacturing and assembly effort because the valve plate is to be mounted close to the transfer channel.
It is an object of the invention to provide a two-stroke engine which is so improved that a reliable assembly of a membrane valve is provided while avoiding additional seal surfaces.
The two-stroke engine of the invention includes a two-stroke engine in a portable handheld work apparatus. The two-stroke engine includes: a crankcase; a cylinder connected to the crankcase; the cylinder having a cylinder wall defining a cylinder bore open to the crankcase; a piston displaceably mounted in the cylinder bore and the piston and the cylinder conjointly defining a combustion chamber; a crankshaft rotatably mounted in the crankcase; the piston being operatively connected to the crankshaft for driving the crankshaft; a mixture inlet in the crankcase; a transfer channel for flow connecting the crankcase to the combustion chamber and the transfer channel having a first end open to the crankcase and a second end communicating with the combustion chamber; a bypass channel for conducting an essentially fuel-free gas; the bypass channel having an opening into the transfer channel at a location between the first and second ends thereof; the transfer channel being configured as a closed channel over the length thereof in the cylinder wall; a membrane valve assembly including a membrane axially inserted into the transfer channel through the first end thereof; and, the membrane being fixed on the cylinder in the region of the first end and projecting in the transfer channel up to in front of the opening.
According to the invention, the transfer channel is configured as an essentially closed channel over its length in the cylinder wall so that a tight channel guidance is provided without additional sealing measures. The membrane valve is to be mounted at the opening of the bypass valve into the transfer channel and is pushed axially into the transfer channel via the open end thereof facing the crankcase and is fixed on the cylinder in the region of the end facing toward the crankcase. The membrane projects up to in front of the opening of the bypass channel and opens and closes the opening in the manner of a check valve. The open end, which is at the crankcase side, defines the assembly opening of the membrane valve. The otherwise necessary additional sealing measures are unnecessary because of this arrangement of the membrane valve.
A two-stroke engine configured in this manner can be operated as a so-called advanced-storage engine or a stratified charge engine, depending upon how the bypass channels, which supply essentially fuel-free gas or air, are switched or controlled.
According to the invention, the membrane of the membrane valve is supported by an essentially stiff membrane carrier which holds the membrane in the open position. The membrane carrier includes a breakthrough, which is arranged at the elevation of the opening, so that the flow in the transfer channel is not hindered by the membrane carrier. The membrane carrier can be fixed in the transfer channel by an attaching screw engaging through the cylinder wall from the outside. The attaching screw advantageously screwed into the membrane carrier.
The sealing seat for the membrane of the membrane valve is formed in the transfer channel. It is advantageous to configure this sealing seat on an insert part, which is manufactured separately from the cylinder and is mounted through the crankcase-side end of the transfer channel. The insert part is advantageously configured as a plate and is advantageously fixed in the cylinder by attachment means engaging in the cylinder from the outside. The insert part lies approximately seal tight on the inner wall of the transfer channel. The plate-shaped insert part extends over the opening of the bypass channel in the longitudinal direction of the transfer channel. A flow opening is provided in the insert part at the elevation of the opening of the bypass channel into the transfer channel and this flow opening connects the bypass channel to the transfer channel. The plate-shaped insert part engages with one end in an assembly slot provided in the cylinder and is fixed therein by attachment means introduced into the cylinder radially from the outside.
In order to ensure a precisely functioning reliable assembly of the membrane valve even by an inexperienced assembler or without sight control, projections are provided on the membrane carrier which engage in assigned openings of the cylinder wall. A first projection can be configured as a stop against rotation and a second projection can be configured as an assembly aid. The attachment screw advantageously engages in the projection forming the assembly aid. For this purpose, the projection, which defines the assembly aid, is configured as a cylinder bushing which lies with an approximate fit in a through bore for the attachment screw. The attachment screw is then screwed into the assembly projection of the membrane carrier and supports itself with its head against an outer annular shoulder of the cylinder wall. In this way, large attachment forces can be developed which ensure a reliable fixing of the membrane valve in the transfer channel.
Advantageously, the projection, which is provided as an assembly aid, can also be used to thread on the insert part. The insert part can, together with the membrane carrier and the membrane, be configured as a preassemblable component. The membrane is preferably held to be clamped between the parts.
In an advantageous embodiment of the invention, the bypass channel is connected via a connecting stub to the transfer channel and the connecting stub is configured as one part with the cylinder.